Girls are sad when a guy they likes are sad because they feel bad for him! major reason. also because you like him and dont want to let him go. you want to be able to see him and or talk to him whenever you get the chance. not not see him and never ever talk to him. most of the time you will know why you are sad over a guy. maybe something serious happened to him and you dont really like it because it is making him sad and depressed and not in the talking mood. even if he knows that you like him.

HOPE IT HELPS!